ci: Test ninja dist in meson jobs

While this isn't critical yet, it might become so at some point, so
let's make sure it keeps working.

Don't set the PIGLIT/XTEST_DIR variables for ninja dist, testing XTS
once against each DDX is enough (as part of ninja test).

Requires git in the docker image.

v2:
* Drop /usr/local/bin/xkbcomp symlink, no longer needed with xkbcomp
  1.4.1 (Peter Hutterer)

.gitlab-ci/debian-install.sh

@@ -5,7 +5,6 @@ set -o xtrace
 
 # Packages which are needed by this script, but not for the xserver build
 EPHEMERAL="
-	git
 	libcairo2-dev
 	libevdev-dev
 	libexpat-dev
@@ -27,6 +26,7 @@ apt-get install -y \
 	ca-certificates \
 	ccache \
 	flex \
+	git \
 	libaudit-dev \
 	libbsd-dev \
 	libcairo2 \